I found this simple circuit of controlling the speed of a DC motor using a potentiometer. I'm wondering if this could still work if I replace the potentiometer with an LDR. I have a variable voltage supply up to about 15V.
The LDR would probably burn.
It can most likely not handle the current needed for the motor.
You could try to make a PWM circuit that reacts on the LDR.
A 10K potentiometer might work.
A 100K potentiometer will not work.
The transistor needs a certain current as input to control the motor speed.
The 10K might already be on the border of a correct working of the circuit.
On the following page you will find some circuits for motor control: Motor control
I only have some simple components such as resistors, capacitors, potentiometer, voltage supply, transistors, etc.. I don't have ICs so I can't use PWM. Could you suggest other ways of doing it aside from the Darlington pair?
Using a straight resistance based control scheme will waste lots and lots of power creating HEAT.
The circuit above uses transistors as a variable resistance to lower voltage. The designer used a 5 inch square chunk of aluminum heatsink material, with fins 1.7 inches tall. The fan was included to keep temperature down. Avg current for the device in use is 10-20 amps and that will result in a 160 degree avg. temp rise. at the transistor mounting points. It was used to provide a slow ramp up of power for bench testing of RC plane motors.(paraphrased from the original descriptions)
Using a PWM scheme with a 555 IC might prove to be easier to do than what you see above, AND it would save lots of power that can be used by the intended load.